The invention relates to apparatus for connecting a telephone switch to a fixed telephone network via a radiotelephone network, with one or more fixed radiotelephone terminals being used to replace a cable link. Each fixed radiotelephone terminal performs functions analogous to those performed by a conventional portable radiotelephone terminal, but it connects the fixed telephone network to a switch instead of connecting it to a single user. The apparatus of the invention may be used to connect a public switched telephone network to a switch serving a hotel, or a village, situated in an isolated region for which laying or a cable or installing some other terrestrial infrastructure would be too costly. The switch serves a group of fixed user terminals by routing calls coming from the public network. Each radiotelephone terminal is used successively for different user terminals because the user terminals are not used continuously. A plurality of radiotelephone terminals can share the same frequency band by means of time division or code division.
The invention relates more particularly to radiotelephone networks of the GSM type, and to those of types derived from the GSM type. Such a network includes a geostationary satellite or Low Earth Orbit (LEO) satellites. A GSM-type network has the following characteristics:
a geographical zone is subdivided into cells, to enable carrier frequencies to be re-used;
a gateway manages the resources of each cell, as regards carrier frequencies and the plurality of channels carried by each carrier frequency; and
at least one switching center of the radio mobile service manages calls, connections with the fixed telephone network, and databases containing: the directory number, the international mobile subscriber identity number, and the location of each mobile subscriber.
Conventional GSM networks are not designed to enable a plurality of subscribers to use the same user terminal. The user personalizes the terminal used by inserting a xe2x80x9cSubscriber Identity Modulexe2x80x9d (xe2x80x9cSIMxe2x80x9d) card into the terminal, which card contains an xe2x80x9cInternational Mobile Subscriber Identityxe2x80x9d (xe2x80x9cIMSIxe2x80x9d) which determines the account to which calls are charged. After an initialization stage, the terminal is designated in the radio messages by a xe2x80x9cTemporary Mobile Subscriber Identityxe2x80x9d (xe2x80x9cTMSIxe2x80x9d) which serves to protect the anonymity of the user. In addition, an authentication procedure uses a secret key contained in the SIM card to prevent any unauthorized users from using the IMSI or the TMSI of another user, when a call is made or when a call is received.
Using the same radiotelephone terminal for different users thus raises the following problems:
it is necessary for it to be possible to issue distinct invoices for user terminals who have used the same fixed radiotelephone terminal;
it is necessary to select automatically that user terminal which is to receive each of the calls successively forwarded by the same fixed radiotelephone terminal to the switch; and
it is necessary to authenticate the radiotelephone terminal that answers a call, even though that terminal is not dedicated to a single user.
Document GB 2 293 524 describes a method making it possible to solve those problems, but that method makes it necessary to modify certain items of equipment in the radiotelephone network used. An object of the invention is to propose apparatus that solves the above-mentioned problems, while making it possible to use a GSM network or a network derived from the GSM type without it being necessary to modify the network.
The invention provides apparatus for connecting a telephone switch to a fixed telephone network or xe2x80x9cpublic switched telephone networkxe2x80x9d via a radiotelephone network or xe2x80x9cpublic land mobile networkxe2x80x9d; a plurality of user terminals being connected to the telephone switch; the apparatus including at least one fixed radiotelephone terminal including means for communicating with the radiotelephone network; said apparatus being characterized in that it includes means for authenticating the radiotelephone terminal by using the same key for all calls received or transmitted via said fixed radiotelephone terminal.
The apparatus characterized in this way does not require a conventional or derived GSM network to be modified. It is necessary merely to allocate the same key to all of the user terminals connected to the switch in question.
In a preferred embodiment, the international mobile subscriber identities (IMSIs) identified by the apparatus are identical apart from n digits, where n is an integer lower than the total number of digits making up an international mobile subscriber identity.
The apparatus characterized in this way is particularly simple to implement, and it makes it possible to use a conventional SIM identity card, merely by masking n digits out of the digits making up the IMSI read from said SIM card: it is possible to distinguish between 10n fixed user terminals by allocating IMSIs to them that are identical apart from the n digits.
In a preferred embodiment, at least one fixed radiotelephone includes means for identifying all of the calls containing an identity belonging to a predetermined set of international mobile subscriber identities, and for identifying all calls containing an identity belonging to a predetermined set of temporary mobile subscriber identities, both of the sets corresponding to the user terminals connected to the switch.
The apparatus characterized in this way does not require the GSM network used to be modified since it makes it possible to use a different IMSI, and a different TMSI, for each user terminal connected to the switch, as if it were a mobile radiotelephone terminal, even though the apparatus actually includes one or more fixed radiotelephone terminals, each of which is used in common by a certain number of user terminals. The GSM network can thus issue a distinct invoice for each fixed user terminal, as it does conventionally for mobile terminals.
In a preferred embodiment, the apparatus further includes means for extracting the directory number of the user terminal that is being called from the SETUP message received for setting up each call, and signalling means for sending said number to the switch so as to select the user terminal that is being called.
The apparatus characterized in this way makes it possible for a called user terminal to be selected directly when a call arrives, i.e. for direct inward dialing to be performed, without it being necessary to modify the GSM or GSM-derived network, even though such a function is not provided for in GSM Standards.
In a preferred embodiment, the apparatus further includes means for periodically performing a location procedure, for all of the international mobile subscriber identities (IMSIs) that correspond respectively to the user terminals that are connected to said switch, or for the corresponding temporary mobile subscriber identities (TMSIs) when such temporary identities have been allocated.
The apparatus characterized in this way makes it possible for the GSM network to determine the geographical zone in which the user terminals connected to a switch are located, by applying the conventional location procedure as used for GSM terminals, without it being necessary to modify the GSM or GSM-derived network.